I 100% agree that thinking antifa should be declared terrorists is outrageous and morally and a ridiculous overstep of the designation, and goes against all kinds of basic human rights (speech/opiniation, assembly, organisation/activism), but claiming antifa isn't a group identity and/or a network because there's no literal organisation called antifa is like claiming there's no such thing as the alt-right because there's no group going around with that literal name.
The point is there is no membership to target with terrorism charges. There is no objective determination as to whether someone is capital-A Antifa or not. Which means it's going to be used to target people who use certain language, match a certain aesthetic, or hold certain political beliefs loosely associated with a vague generalization of what Antifa is.
The same way War on Terror laws were not drafted with strict specifications as to how someone is determined to be a terrorist and what is a terrorist activity, which enabled those laws to be used to make leftist activists targets for surveillance and harassment.
Also, I think your comparison to the alt-right isn't very valid, as their group affiliation is defined by sharing a whole set of political beliefs and goals. They have a shared vision for how they would like society to be that has far-reaching implications into every aspect of life, and consequently the definition of what constitutes an alt-right person is reasonably narrow. Still not narrow enough that labeling them a terrorist organization would be legally sensible, mind, but much closer.
But antifa is not a whole set of political beliefs, and the only shared goal is to stop the growth of fascism. Affiliation with capital-A Antifa is impossible to narrowly define. There may be groups and networks formed around that goal, and they may even use names for themselves that include the word Antifa. But there is no broader scale organization, and no broader set of shared political beliefs. No vision for the world to pursue beyond "no fascism pls". Any attempt to define affiliation with capital-A Antifa would also have to blanket over every single person who supported the Allies in WW2. Not exactly useful as far as legal standards of definition go.
It's more like calling Environmentalism an organization. There's groups and networks of environmentalists. There's a shared identity in caring about the environment. But I don't think any sane person would call Environmentalism and organization for any legal purpose in a way that makes any sense. Any legal attempt would result in anyone who expresses concern for the environment being potentially subject to those laws, which is a vast range of varying people who will mostly be not affiliated with each other in any meaningful fashion at all.
And that's exactly the intended result with trying to claim Antifa is an organization. Cast as wide a net as you can get away with. Drag in every fish you can. Keep the ones you want. Make any fish you don't like go extinct.